The United States has some of the richest deposits of shale gas in the world. U.S. lawmakers expressed concern that some chemicals used during fracking, the process used to extract natural gas from shale rock formations, are carcinogenic or otherwise harmful and could get into groundwater supplies.
The EPA in February said eight of the nine companies that practice fracking voluntarily handed over information to help the agency examine the practice. It had to subpoena U.S. energy company Halliburton to hand over its records.
"This is an important part of a process that will use the best science to help us better understand the potential impacts of hydraulic fracturing on drinking water," Paul Anastas, assistant administrator for EPA's office of research and development, said in a statement about the study.
Operators in Michigan under new measures passed last month are required to post a Material Safety Data Sheet listing characteristics of the chemical additives used in fracking fluid. The operators must also submit records related to volumes, rates and pressures associated with fracking.
The Michigan Department of Environmental Quality said fracking has been practiced in the state without incident for more than 70 years.
Congress ordered the EPA fracking study two years ago. Final results of the EPA's study are expected by 2014.